1. Field of the invention
The present invention relates to a superconductor/insulator metal oxide hetero structure for an electric field tunable microwave device, and particularly to a structure which realizes, a novel microwave device.
2. Description of related art
Electromagnetic waves called "microwaves" or "millimetric waves" having wavelengths range from tens of centimeters to millimeters can be theoretically said to be merely a part of an electromagnetic wave spectrum, but in many cases, have been considered from an engineering viewpoint to be a special independent field of the electromagnetic wave spectrum, since special and unique methods and devices have been developed for handling these electromagnetic waves.
Microwave properties of any material can be conveniently expressed in terms of a complex parameter, surface impedance that describes the interaction between the material and any electromagnetic radiation incident upon it. The real and imaginary components of the surface impedance are called surface resistance and surface reactance, respectively. Surface resistance is the quantity that is proportional to the microwave energy dissipation induced in the material whereas surface reactance is related to the microwave energy stored in the material.
For most passive microwave devices, it is desirable to have low energy dissipation, i.e. low surface resistance, so that microwave signals can be sent efficiently and to longer distances. Also, for the transmission of microwave signals in most applications with multifrequency components, it is desirable to have a transmission medium with negligible or no dispersion; in other words frequency independent energy storage, i.e. surface reactance in the system.
In general, superconductors are theoretically expected and experimentally shown to have lower surface resistance and nearly frequency independent surface reactance, i.e. much lower dispersion than normal conductors at microwave frequencies and certain cryogenic temperatures. This makes superconductors attractive for most passive microwave device applications.
In addition, the oxide superconductor material (high T.sub.c copper oxide superconductor) which has been recently discovered in study makes it possible to realize the superconducting state by low cost liquid nitrogen cooling. Therefore, various microwave components using an oxide superconductor have been proposed.
For active microwave device applications, in addition to the above mentioned requirements, it is necessary to modulate the surface impedance of the device by an independent external bias. Among various methods to modulate the microwave response of a circuit, electric field induced modulation has clear advantages such as low energy consumption input-output current isolation and high input resistance.
A. M. Hermann et al. showed in Bulletin of Am. Phys. Soc. Vol. 38, No. 1, pp. 689 (1993), a tunable microwave resonator comprising two superconducting electrodes of Tl--Ba--Ca--Cu--O thin films and an insulating layer of Ba.sub.o.1 Sr.sub.0.9 TiO.sub.3 between the superconducting electrodes. In this microwave resonator, the resonant frequency is controlled by a dc bias voltage applied to the resonator. In the resonator, the dc bias voltage changes the dielectric constant of Ba.sub.o.1 Sr.sub.0.9 TiO.sub.3 so that a 1.5% shift in resonant frequency can be obtained. However, the shift in resonant frequency is only to the changes in the properties of the dielectric medium.
David Galt et al. also showed a tunable microwave resonator of a different structure in Bulletin of Am. Phys. Soc. Vol. 38, No. 1, pp. 840 (1993)
In Bulletin of Am. Phys. Soc. Vol. 38, No. 1, pp. 838 (1993), Alp T. Findikoglu et al. showed that both the resonant frequency and the quality factor of a resonator can be controlled by a dc bias applied between two superconducting layer across a dielectric layer, all forming part of the resonator. It is shown here that the microwave response is modulated through changes in both the superconducting properties of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. oxide superconductor (where 0.ltoreq..delta..ltoreq.0.5) and dielectric properties of SrTiO.sub.3.